1. Field of the Invention
The present invention relates to a semiconductor laser, and more specifically to a high output semiconductor laser having a reduced power consumption and an improved productivity.
2. Description of the Prior Art
There has been known in the art a technique in which an n-type clad layer, an active layer and a p-type clad layer are grown in sequence on a semiconductor substrate to form a laser chip for an oscillating laser beam, and such a prior art technique is disclosed in U.S. Pat. No. 5,016,252, Japanese Laid-Open Patent Publication No. 2-71574, and Japanese Laid-Open Patent Publication No. 61-247084.
There has been also provided a semiconductor laser in which more than two laser chips are stacked to obtain a higher output. An example of such a laser is shown in FIG. 10. The semiconductor laser in FIG. 10 has two laser chips 8 stacked one on the other, with solder 8a provided therebetween to bond the chips 8 to each other.
Each of the semiconductor laser chips 8 is formed by growing in sequence an n-Al.sub.x Ga.sub.1-x As clad layer 82, an undoped GaAs active layer 83, a p-Al.sub.x Ga.sub.1-x As clad layer 84 and a p.sup.+ -GaAs contact layer 85 on an n.sup.+ -GaAs semiconductor substrate 81. In the drawing, numeral 80 designates an n-side electrode, 87 a p-side electrode and 86 a current blocking layer. When voltage is applied to the electrodes 80 and 87, carriers are injected into the active layer 83 from the clad layers 82 and 84, and recombination of the carriers converts the active layer 83 into a light radiating region.
When such a two-chip stacked semiconductor laser is driven by pulses having a pulse width of 50 ns, a duty ratio of 0.025% and an input current of 25A, a peak output of 30W is obtained.
Stacking of semiconductor laser chips 8 is not limited to the above mentioned two chips but may include more chips, and a device including six chips stacked to obtain a peak output of 100W is commercially available.
However, there are several disadvantages when two or more semiconductor laser chips 8 are stacked with solder 8a interposed therebetween to bond the chips, as described above, such as a defective light radiating pattern due to a poor bonding accuracy, a destruction of the semiconductor laser chips 8 due to thermal damage during the bonding process thereof and an increase of labor cost required in the bonding process of the semiconductor laser chips 8.
Though the n.sup.+ -GaAs substrate 81 of the semiconductor laser chip 8 has a relatively low specific resistance (about 1.times.10.sup.-3 .OMEGA.cm), the flow of driving current of several tens of amps results in a power consumption of several hundreds of mW caused by the n.sup.+ -GaAs substrate 81. Thus, stacking of the semiconductor laser chips 8 by means of solder 8a provided therebetween causes the n.sup.+ -GaAs substrates 81 to be stacked, resulting in an increased power consumption.